In ink jet recording using water-color ink, characteristics such as quick absorption of ink, reproduction quality, and clarity and resolution of the ink coloring material on a recording medium are required for an ink jet recording medium. Conventionally, various proposals have been made to provide a recording medium with such characteristics. Japanese examined patent publication Sho 60-27588 (1985) describes a recording medium whose ink absorbability is improved by having no coating on an unsized paper.
Such a recording medium, however, has problems that in spite of quick ink absorbability, the ink penetrates deeply into the paper, resulting in poor ink reproduction quality and concentration. Penetration of ink in the direction orthogonal to thickness is also quick, and this spreading means that high resolution cannot be obtained. Further, feathering, and bleeding following the fibers occurs, which provides very poor reproduction.
To solve these defects, for example, Japanese unexamined patent publication Sho 58-72495 (1983) describes a recording medium having a thick coating of high ink absorbability on a substrate of high sizing degree.
As such a material requires a high proportion of pigment in the coating and a thick coating, its feel and appearance are different from the plain paper used for office copying purposes and uncoated printing paper. Further, if it is written on by a writing tool having a hard point, the coating is broken and it is difficult to write on; additionally powdery peeling may be caused by folding or friction, causing feeding failure by the attachment of loose coating to a feed roller or head clogging.
To solve these defects, Japanese unexamined patent publication Sho 63-1583 (1988) describes a recording medium having a thin coating, a Steckigt sizing degree of the substrate of not more than 5 seconds, a high bulk density, with a pore ratio of not more than 50 percent, and a specified filler ratio. Japanese unexamined patent publication Hei 2-16079 (1990 ) describes a recording medium having a roughness index of the coating of at least 10 ml/m.sup.2 and a BeKK smoothness of the coating of not more than 20 seconds.
The recording medium of Japanese unexamined patent publication Sho 63-1583 (1988), however, has excellent ink absorbability, but for color reproduction is liable to have inter-color bleeding caused by ink penetration across the borders between portions of adjoining colors and superimposed portions of different colors.
In the recording medium of Japanese unexamined patent publication Hei 2-16079 (1990), attempts were made to improve the inter-color bleeding by reducing the amount of coating and roughening the coating surface, but especially in high speed and high ink jet density recording, there is only a short interval between printing the first color and printing the second color and the amount of ink for a unit area is high, so surface roughening is not sufficient to achieve satisfactory quality. Further, surface roughening allows the ink to enter the concave portions of the medium, and makes the spreading of ink drops in the medium surface uneven, which reduces the image reproduction.